Candida is the most commonly identified causative agent of oral or vaginal thrush. However, over the last few decades Candida has emerged as a significant cause of life-threatening infections in hospital patients. Ironically the increasing incidence of these “invasive” or “systemic” Candida infections has been advances in modern medicine. Patients that are now surviving major injuries, surgery, cancers and organ transplants are vulnerable to life-threatening Candida infections. In the United States, Candida is now the forth most common cause of blood infections in hospitals.
The major problem with systemic Candida infections is that there are few definitive clinical signs or symptoms. Treatment is largely based on suspicion rather than a definitive diagnosis. Even with the availability of anti-fungal drugs such as fluconazole a high mortality rate (30 to 70%) is associated with systemic Candida infections. The high rate of mortality is largely due to the rapid onset of infection and a rapidly fatal outcome. Without an accurate diagnosis the infection often goes unnoticed until it is too late to effectively treat. This has led to a comment by clinicians that Candida infections are usually diagnosed at autopsy. Accordingly, there is a need for a rapid diagnostic assay that is capable of early diagnosis of Candida infection so that appropriate treatment may be instituted thereby reducing the mortality rate.
The main difficulty in the diagnosis of Candida infections is that being a commensal, mere isolation of Candida from body surfaces, or orifices, is not diagnostic of an infection. Culture of Candida from blood or deep tissue is still the main method of diagnosis of systemic Candida infections. However, it can take several days for a culture to become positive, by then it may be too late to effectively treat the infection. Also, false positives may occur due to contamination from superficial body sites. Of more importance, is the observation that in up to fifty percent of autopsy proven cases of systemic candidiasis, blood cultures were negative and therefore of no diagnostic value.
Nuclear magnetic resonance (NMR) and radioisotope scanning have been used to detect Candida infections in tissues and organs. However, those methods are not useful for early diagnosis.
Recently analysis of the Candida metabolite arabinitol was proposed as a diagnostic tool. However, as arabinitol is produced by the human body, further clinical studies have cast doubt on its value.
The polymerase chain reaction (PCR) has also been used in the diagnosis of invasive Candida infections. However, PCR has not established itself as a useful diagnostic method for Candida for the same reasons as outlined above ie Candida is a ubiquitously present microorganism and false positives, due to superficial contamination, are prevalent.
Immunoassays are the established procedures for the diagnosis of many types of infectious diseases. Immunoassays have the advantage that they are rapid and have a standardised assay format. Immunoassays can be designed to either detect Candida antigens, or host antibodies reactive against Candida antigens. Several immunoassays are commercially available for the detection of Candida antigens in sera or other body fluids. However, these assays lack either sensitivity or specificity or both.
Immunoassays have been developed based on the detection of immunodominant Candida antigens. Candida mannan is a highly immunogenic cell wall antigen. However, as Candida is a commensal, most individuals have antibody to Candida mannan, so its usefulness in the diagnosis of systemic infection is limited. The applicant has now surprisingly found that a more discriminatory assay for Candida than previously used is the detection of cytoplasmic antigen. The advantage of this diagnostic assay is that antibody to this cytoplasmic antigen is only produced in response to an actual infection. The applicant has further demonstrated that the use of a combination of cytoplasmic antigens with other antigens is very predicative of Candida infection.
Accordingly, the present invention overcomes or at least alleviates the problems normally associated with diagnosing Candida infection.